Polymer composition, membrane comprising the same, process for production thereof and use thereof.
The present invention relates to a polymer composition which is suitable in particular for producing membranes, and also to the use of these membranes in fuel cells, in high-performance capacitors, in dialysis equipment and in ultrafiltration.
Fuel cells are electrochemical energy converters which feature in particular a high level of efficiency. Among the various types of fuel cells, polymer electrolyte fuel cells (PEM hereinafter) feature high power density and a low weight to power ratio.
Conventional fuel cells generally operate using membranes based on fluorine-containing polymers, for example using the material Nafion(copyright).
For the commercialization of fuel cell technology, in particular for relatively large-scale applications, it is necessary to reduce the production costs of the materials used without thereby having to accept sacrifice of performance compared with the materials conventionally used.
Proton-conducting membranes based on sulfonated polyether ketones are known, for example from a report article by A. Steck in Proc. 1st Inter. Symp. on New Materials for Fuel Cell Systems, Montreal 1995, pp. 74 or from an article by C. A. Linkous et al. in Int. J. Hydrogen Energy, Vol. 23, No. 7, pp. 525-9 (1998). WO-A-96/29359 and WO-A-96/29360 describe polymer electrolytes made from sulfonated aromatic polyether ketones, and the production of membranes from these materials.
EP-A-0152161 describes polyether ketones (PEK hereinafter) composed predominantly of the repeat unit xe2x80x94Oxe2x80x94Arxe2x80x94COxe2x80x94Arxe2x80x94Arxe2x80x94, and molded structures produced therefrom.
Sulfonated, strictly alternating polyether ketones with the repeat unit xe2x80x94Oxe2x80x94Arxe2x80x94COxe2x80x94Arxe2x80x94 are described in J. Polym. Sci.: Vol. 23, 2205-2222, 1985. The structure of the polyether ketones here is a result of electrophilic attack, and not nucleophilic attack as described in EP-A-0152161. The polymers were sulfonated by sulfur trioxide using triethyl phosphate in dichloroethane. Another sulfonation method used in this literature reference is chlorosulfonation using chlorosulfonic acid. However, in this method, depending on the degree of sulfonation, molecular-weight degradation is also observed. The amidation of the acid chloride follows on. A possible application sector given for polymers of this type is use as ion exchanger or as salt remover. Use in fuel cells is not described. Property profiles which suggest use in fuel cells are also absent.
EP-A-0688824 mentions membranes also for use in electrochemical cells and made from homogeneous polymer alloys based on sulfonated aromatic polyether ketones and polyether sulfones and a third, hydrophilic polymer.
WO-A-98/07164 has disclosed mixtures made from high-molecular-weight acids (sulfonated polyether ketones, for example) and high-molecular-weight bases (polybenzimidazoles, for example). However, there is no indication here of the combinations of properties. required to permit operation in fuel cells. The invention described in this document is also directed toward a water-free conductivity mechanism brought about by acid/base interaction, and therefore permitting use of these materials at temperatures above 100xc2x0 C. and at atmospheric pressure.
The application of polybenzimidazoles in the fuel cell has previously been described by Savinell et al. in J. Electrochemical Soc., 141, 1994, pp. L46-L48. Mixtures of different polymers with polybenzimidazoles are also known, e.g. from U.S. Pat. No. 5,290,884.
The suitability of nonfluorinated aromatic polymers, a class which includes aromatic polyether ketones, for use in fuel cells is questioned in the literature (A. Steck, Proc. 1st Inter. Symp. on New Materials for Fuel Cell Systems, Montreal 1995, pp 74).
Modifying the properties of polymeric materials by admixing other components is a well known process. However, the property profile of polymer mixtures is difficult to predict. It is doubtful that there is any theory which reflects the complex nature of polymer-polymer interactions (Macromolecules, Vol. 16, 1983, pp. 753-7).
The invention provides compositions from which high-performance membranes can be produced using cost-effective materials. The novel compositions moreover provide a material whose performance exceeds that of the standard fluorinated materials conventionally used. The novel compositions also provide a material from which membranes with good mechanical properties, and also excellent proton conductivity, can be produced.
This combination of properties was not to be expected and does not arise with other polymer mixtures. For example, with compositions made from sulfonated polyether ketone and polyether sulfone it is found that addition of even small amounts of polyether sulfone leads to a marked fall-off of proton conductivity of the membranes made from this material.
The present invention provides compositions comprising from 30 to 99.5% by weight of a sulfonated aromatic polyether ketone which has an ion-exchange capacity of from 1.3 to 4.0 meq (xe2x80x94SO3H)/g of polymer and from 0.5 to 70% by weight of a polybenzimidazole.
The ion-exchange capacity (hereinafter also xe2x80x9cIECxe2x80x9d) is determined by elemental analysis of the washed and dried polymer via determination of the ratio of carbon to sulfur (C/S quotient).
For the purposes of this invention, aromatic polyether ketones are any polymer which has structural units xe2x80x94Arxe2x80x94Oxe2x80x94 and xe2x80x94Arxe2x80x94COxe2x80x94, where Ar is an aromatic radical. These structural units may have been linked to one another in a variety of ways, particularly in the para position. Following widely used terminology the first unit is termed xe2x80x9cExe2x80x9d (ether) and the second unit xe2x80x9cKxe2x80x9d (ketone). Depending on the sequence of the ether units and ketone units, a distinction can be made between, for example, PEK, PEEK, PEKK and PEEKK types. All of these types of polymer are included in the term polyether ketones for the purposes of this invention. The sulfonated aromatic polyether ketones used according to the invention may be any desired polymers, for example PEEK, PEKK, PEEKK or in particular PEK, as long as they have the ion-exchange capacity defined above.
Particular preference is given to compositions in which the sulfonated polyether ketone has the repeat unit of formula I
xe2x80x94[Ar1xe2x80x94Oxe2x80x94Ar2xe2x80x94CO]xe2x80x94xe2x80x83xe2x80x83(I),
where Ar1 and Ar2, independently of one another, are bivalent aromatic radicals, unsubstituted or substituted by one or more monovalent organic groups inert under usage conditions, and where at least a portion of the radicals Ar1 and Ar2 have substitution by radicals of the formula xe2x80x94(SO3)wM, where M is a metal cation of valency w, an ammonium cation or in particular hydrogen, and w is an integer, in particular 1 or 2. M is preferably a cation of an alkali metal or of an alkaline earth metal.
If any radicals are bivalent aromatic radicals, these are mono- or polynuclear aromatic hydrocarbon radicals or heterocyclic-aromatic radicals which may be mononuclear or polynuclear. In the case of heterocyclic-aromatic radicals, these have in particular one or two oxygen, nitrogen or sulfur atoms in the aromatic radical.
Polynuclear aromatic radicals may have been fused with one another or bonded via Cxe2x80x94C bonds or via bridging groups, such as xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94SO2xe2x80x94 or xe2x80x94CnH2nxe2x80x94, where n is an integer from 1 to 10.
In the case of the bivalent aromatic radicals, the location of the valence bonds may be in the para position or in a comparable coaxial or parallel position or in the meta position or in a comparable angled position relative to one another.
The valence bonds which are in a coaxial or parallel position relative to one another point in opposite directions. An example of coaxial bonds which point in opposite directions is given by the bonds in 4,4xe2x80x2-biphenylene. An example of parallel bonds which point in opposite directions is given by the bonds in 1,5- or 2,6-naphthalene, while the 1,8-naphthalene bonds are parallel and point in the same direction.
Examples of preferred bivalent aromatic radicals Ar1 and Ar2, the location of whose valence bonds is in the para position or in a comparable coaxial or parallel position relative to one another, are given by the mononuclear aromatic radicals with free valences in the para position relative to one another, in particular 1,4-phenylene, or fused binuclear aromatic radicals with parallel bonds which point in opposite directions, in particular 1,4-, 1,5- and 2,6-naphthylene, or binuclear aromatic radicals linked via a Cxe2x80x94C bond and having coaxial bonds which point in opposite directions, in particular 4,4xe2x80x2-biphenylene.
The valence bonds which are in the meta position or a comparable angled position relative to one another are arranged at an angle.
Examples of preferred bivalent aromatic radicals Ar1 and Ar2, the location of whose valence bonds is in the metaposition or in a comparable angled position relative to one another, are given by mononuclear aromatic radicals with free valences in the metaposition relative to one another, in particular 1,3-phenylene, or fused binuclear aromatic radicals with bonds which point at an angle relative to one another, in particular 1,6- and 2,7-naphthylene, or binuclear aromatic radicals linked via a Cxe2x80x94C bond and having bonds which point at an angle relative to one another, in particular 3,4xe2x80x2-biphenylene.
Preferred radicals Ar1 and Ar2 are 1,3-phenylene or in particular 1,4-phenylene.
The aromatic radicals of the polymers used according to the invention may have substitution by inert groups. For the purposes of the present invention, these are substituents which do not adversely affect the application under consideration.
Examples of substituents of this type are alkyl, alkoxy, aryl, amino, alcohol, ether, sulfonyl, phosphonyl, acyl, nitro, carboxylic acid, carboxylic ester or carboxamide groups, or halogen.
For the purposes of the present invention, alkyl groups are branched or preferably straight-chain alkyl radicals, for example alkyl having from one to six carbon atoms, in particular methyl.
For the purposes of the present invention, alkoxy groups are branched or preferably straight-chain alkoxy radicals, for example alkoxy radicals having from one to six carbon atoms, in particular methoxy.
For the purposes of the present invention, amino groups are radicals of the formula xe2x80x94NH2, xe2x80x94NHR1 or xe2x80x94NR1 R2, where R1 and R2, independently of one another, are alkyl radicals or aryl radicals, preferably methyl. For the purposes of the present invention, alcohol groups are radicals of the formula xe2x80x94OH.
For the purposes of the present invention, ether groups are radicals of the formula R1 xe2x80x94Oxe2x80x94, where R1 is as defined above.
For the purposes of the present invention, sulfonyl groups are radicals of the formula xe2x80x94SO2R1, where R1 is as defined above.
For the purposes of the present invention, phosphonyl groups are radicals of the formula xe2x80x94P(OR3)3, where the radicals R3, independently of one another, are hydrogen, alkyl or aryl.
For the purposes of the present invention, acyl groups are radicals of the formula xe2x80x94COxe2x80x94R3, where R3 is as defined above.
For the purposes of the present invention, carboxylic acid groups are radicals of the formula xe2x80x94COOH.
For the purposes of the present invention, carboxylic ester groups are radicals of the formula xe2x80x94COOR1, where R1 is as defined above.
For the purposes of the present invention, carboxamide groups are radicals of the formula xe2x80x94CONH2, xe2x80x94CONHR1 or xe2x80x94CONR1 R2 where R1 and R2 are as defined above.
If any radicals are halogen, these are fluorine or bromine, for example, or in particular chlorine.
Preference is given to compositions in which Ar1 and Ar2 are naphthylene or in particular phenylene.
Preference is given to compositions in which Ar1 and Ar2 have substitution by from one to four amino, alcohol, ether, alkyl, aryl, sulfonyl, phosphonyl, acyl, nitro, carboxylic acid, carboxylic ester and/or carboxamide groups, and/or in which the nitrogen atoms of the polybenzimidazole have substitution by these groups.
Particular preference is given to compositions in which the sulfonated polyether ketone has an ion-exchange capacity of from 1.6 to 2.9 meq (xe2x80x94SO3H)/g of polymer.
For the purposes of the present invention, polybenzimidazoles are any polymer which has repeat structural units of the formula II 
where Arxe2x80x3 is a tetravalent aromatic radical, Arxe2x80x2 is a bivalent aromatic radical and R is hydrogen or an inert monovalent organic radical.
The bivalent aromatic radicals Arxe2x80x2 may, as for Ar1 and Ar2, be mono- or polynuclear aromatic hydrocarbon radicals or heterocyclic-aromatic radicals, which may be mono- or polynuclear. The location of the valence bonds in Arxe2x80x2 may be in the para position or in a comparable coaxial or parallel position or in the meta position or in a comparable angled position with respect to one another. Examples of radicals Arxe2x80x2 have already been given above when describing the radicals Ar.
Preferred radicals Arxe2x80x2 are 1,3-phenylene and in particular 1,4-phenylene.
The tetravalent aromatic radicals Arxe2x80x3 may likewise be mono- or polynuclear aromatic hydrocarbon radicals or heterocyclic-aromatic radicals, which may be mono- or polynuclear. The valence bonds in Arxe2x80x3 are in each case in a pair arrangement, to allow the two imidazole rings to form.
Preferably, the location of the two valence bonds is in each case in the ortho position with respect to one another, and in turn the location of these pairs is in opposite positions on the aromatic ring or on the ring system.
Examples of preferred radicals Arxe2x80x3 are 1,2,4,5-phenylene and 3,4,3xe2x80x2,4xe2x80x2-biphenylene.
Other polybenzimidazoles and preferred radicals Arxe2x80x3 and Arxe2x80x2 are described in U.S. Pat. No. 5,290,884, the Description of which is incorporated into the present Description.
The aromatic radicals Arxe2x80x2 and/or Arxe2x80x3 of the polybenzimidazoles used according to the invention may have substitution by inert groups. For the purposes of the present invention, these are substituents which do not adversely affect the application under consideration. Examples of these have been listed above for the sulfonated polyether ketones.
Particular preference is given to a polybenzimidazole of the formula II, in which Ar is 1,2,4,5-phenylene or 3,4,3xe2x80x2,4xe2x80x2-biphenylene, Arxe2x80x2 is 1,3- or 1,4-phenylene and R is hydrogen.
Particular preference is given to compositions in which the proportion of the polybenzimidazole is selected depending on the degree of sulfonation of the sulfonated polyether ketone. It has been found that there is an ideal mixing ratio of sulfonated polyether ketone to polybenzimidazole, depending on the ion-exchange capacity of the polyether ketone used. Membranes produced from polymer mixtures of this type have an ideal combination of properties comprising modulus of elasticity at 80xc2x0 C. in water, swelling behavior at 80xc2x0 C. and proton conductivity.
For sulfonated PEK types of the formula I it has been found that the proportion of the polybenzimidazole should preferably be selected depending on the degree of sulfonation of the sulfonated polyether ketone according to formula III below:
percent by weight of polybenzimidazole =9.4xc3x97xe2x88x9212.4xc2x1(9.4 xc3x97xe2x88x9212.4)xc3x970.5xe2x80x83xe2x80x83(III).
x here is the ion-exchange capacity of the sulfonated polyether ketone in meq (xe2x80x94SO3H)/g of polymer.
The molecular weight of the polymers used in the novel compositions must be sufficient to allow polymer solutions to form from which moldings, preferably membranes, can be constructed.
The sulfonated polyether ketones preferably have molar masses (number-average) in the range from 45,000 to 70,000 g/mol, determined by gel permeation chromatography in NMP using salts with polystyrene calibration.
The polybenzimidazoles preferably have an intrinsic viscosity in the range from 0.8 to 1.2, measured at 25xc2x0 C.
The novel compositions are particularly suitable for producing membranes with excellent performance characteristics.
The invention also provides membranes comprising the compositions defined above.
The novel membranes usually have a thickness of at least 5 xcexcm, preferably more than 10 xcexcm, particularly preferably from 10 to 100 xcexcm. For applications in the fuel cell the thickness of the membranes is generally at least 30 xcexcm, and for applications as a dielectric in capacitors the thickness of the membranes is generally at least 5 xcexcm.
It is preferable to use polymer solutions with different viscosities, depending on the desired thickness of the membrane. For membranes of thickness of from 5 to 50 xcexcm it is preferable to use polymer solutions with a viscosity of from 500 to 2000 mPas (measured at 80xc2x0 C. on a solution of the polymers in the relevant solvent). For membranes of from 10 to 100 xcexcm thickness it is preferable to use polymer solutions with a viscosity of from 1500 to 5000 mPas (measured at 80xc2x0 C. on a solution of the polymers in the relevant solvent).
The resultant membranes were tested mainly with respect to their mechanical stability in the dry state and in the wet state, their proton conductivity and their performance in the fuel cell.
It has been found that the novel membranes feature excellent electrical properties. These include an ion conductivity of not less than 50 mS/cm (measured in contact with liquid water at room temperature with the aid of 4-pole impedance spectroscopy at a phase angle |"THgr"| less than 1xc2x0).
Proton conductivity in the range from 120 to 200 mS/cm at 80xc2x0 C. (measured by impedance spectroscopy using the 4-pole method in pure water) has been found, together with excellent mechanical properties. The novel membranes feature excellent mechanical properties. These include a modulus of elasticity of at least 600 MPa in the dry state at 23xc2x0 C. and 50% relative humidity, a modulus of elasticity of at least 90 MPa in water at 60xc2x0 C., a modulus of elasticity of at least 50 MPa in water at 80xc2x0 C. and an ultimate elongation of more than 200%. The moduli of elasticity here were in each case determined as a gradient of the tangent at 1.2 MPa.
It has been found, therefore, that there is an increase in mechanical stability. For example, the modulus of elasticity determined in water (gradient of the tangent at 1.2 MPa) rises to a value of 350 N/mm2 at 80xc2x0 C. In contrast to this, the modulus of elasticity determined for pure materials was only from 4 to 5 N/mm2. It is highly surprising that results of this type have not been found with mixtures using PES and PEEK (IEC 1.54 mmol/g of polymer).
The novel membranes also feature excellent resistance to boiling water. For example, it has been found that novel membranes based on sulfonated PEK remain mechanically stable after 72 hours of treatment in boiling water at 100xc2x0 C.
The novel membrane preferably has a residual content of solvent of less than 0.5% by weight. It has been found that membranes made from sulfonated PEEK with an IEC of at least 1.5 meq (xe2x80x94SO3H)/g of polymer (based on Victrex 450 PF) are stable for only from about 2 to 3 hours in boiling water. Surprisingly, membranes made from sulfonated polyether ketones, e.g. based on Victrex PEK and having a comparable IEC, are stable for more than 50 h in boiling water. The invention therefore also provides a polyether ketone of PEK type which has an ion-exchange capacity of from 1.3 to 4.0 meq (xe2x80x94SO3H)/g of polymer, and also a membrane produced therefrom.
It has also been found that the polyether ketone polymer backbone structure, which is electron-deficient due to the absence of xe2x80x94Oxe2x80x94Arxe2x80x94Oxe2x80x94 units, appears to be particularly suitable for fuel-cell applications.
Sulfonated polyether ketones having the repeat unit xe2x80x94Oxe2x80x94Arxe2x80x94COxe2x80x94Arxe2x80x94 can currently be produced on an industrial scale up to an IEC of about 4.0 meq (xe2x80x94SO3H)/g of polymer.
It has been found that membranes made from highly sulfonated polymers of this type or membranes made from compositions comprising highly sulfonated polymers of this type and polybenzimidazoles are particularly useful for fuel cells with little or no humidification and also for so-called super-caps, i.e. capacitors with extremely high capacitance. The membrane may also be used in electrodialysis or in ultrafiltration. The invention also provides the use of the membranes for these applications.
The invention further provides a process for producing the membranes described above. The process comprises
a) preparing a solution comprising from 30 to 99.5% by weight of a salt of a sulfonated polyether ketone and from 0.5 to 70% by weight of a polybenzimidazole by dissolving the two polymers in a suitable organic solvent, in particular dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide or N-methyl-2-pyrrolidone, and
b) shaping this solution by processes known per se, such as casting, doctoring, spraying or centrifugal processes, to give a membrane.
Mixtures of polybenzimidazoles and sulfonated polyether ketones tend to gel spontaneously as a result of the acid-base interaction which takes place, and it is therefore difficult or impossible to process them further to give sheet-like structures, such as membranes. This also applies at elevated temperature.
A homogeneous solution of sulfonated polyether ketones and polybenzimidazoles can be prepared by using polybenzimidazole and the salts, preferably the Li, Na, K or ammonium salts, of the sulfonic acids in dry organic solvents, preferably DMSO, DMF, DMAc or NMP. The resultant solution of the blend can be applied to a support and dried at temperatures up to 160xc2x0 C.
Despite the diversion described via the salts of the sulfonic acid, the production technology described is of great interest, since it allows production of membranes having the combined properties of high proton conductivity and high modulus of elasticity at 80xc2x0 C. in water, and also low swelling.
Phase-inversion membranes for use in ultrafiltration are usually produced by introducing the solution of the polymer or of the polymer mixture (e.g. sulfonated PEK/PBI in NMP or in DMAc) and precipitating in a non-solvent (e.g. water).
The membrane is usually converted into the acid form of the sulfonic acid by conditioning using a dilute acid, preferably a dilute mineral acid, such as an acid of from 0.1 to 20% strength (sulfuric acid, phosphoric acid or nitric acid). At the same time, ionic (salts) and organic (solvent residues) impurities are removed by this treatment.
Another way is to convert the ammonium form of the membrane into the acid form by thermal cleavage of the ammonium group (liberating NH3).
If desired, the membrane obtained from the pretreatment described above may then be washed with water.
The membrane may be then be dried by heating, until, for example, the residual content of solvent is less than 0.5% by weight. Another preferred version of the novel process provides the production of a membrane in which the solution comprising from 30 to 99.5% by weight of the salt of the sulfonated polyether ketone and from 0.5 to 70% by weight of the polybenzimidazole is introduced into an absorbent web, and the solvent is then removed by evaporation.
The novel membranes may be further processed wet or dry.